Lithium-ion secondary batteries are secondary batteries having high charge and discharge capacity and capable of outputting high power. The lithium-ion secondary batteries are now mainly used as power sources for portable electronic devices and are promising power sources for electric vehicles to be widely used in future. A lithium-ion secondary battery has an active material capable of absorbing and releasing lithium (Li) at each of a positive electrode and a negative electrode. The lithium-ion secondary battery works by moving lithium ions in an electrolytic solution provided between these two electrodes. In such a lithium-ion secondary battery, lithium-containing metal composite oxide such as lithium-cobalt composite oxide is mainly used as an active material for a positive electrode, and a carbon material having a multilayer structure is mainly used as an active material for a negative electrode.
The lithium-ion secondary batteries are demanded to have higher capacity and are under study for positive electrode potential to rise a voltage. However, when used at a high voltage, the lithium-ion secondary batteries have a problem that battery characteristics drastically deteriorate after repeated charge and discharge. This is supposed to be caused by oxidation decomposition of electrolytic solutions or electrolytes around positive electrodes when the batteries are charged.
That is to say, a decrease in capacity is considered to be caused by consumption of lithium ions by oxidation decomposition of electrolytes around positive electrodes. Moreover, a decrease in output power is considered to be caused because decomposed materials of electrolytic solutions or electrolytes deposit on pores inside the electrodes and separators and exhibit resistance to lithium-ion conduction. Therefore, in order to solve these problems, decomposition of the electrolytic solutions needs to be suppressed.
Japanese Unexamined Patent Application Publication No. H11-097,027, Japanese Unexamined Patent Application Publication (Translation of PCT International Application) No. 2007-510,267 and the like disclose nonaqueous secondary batteries each having a positive electrode having a coating layer comprising an ion-conductive polymer on a surface thereof. Formation of such a coating layer suppresses degradation, such as elution and decomposition, of a positive electrode active material.
These publications, however, do not describe evaluation of the batteries when charged at a high voltage of 4.3 V or more, and it is unclear whether the batteries withstand use at such a high voltage. The coating layers substantially have thicknesses on a micrometer order and exhibit resistance to lithium-ion conduction. Spray coating and one-time dipping coating are employed for forming these coating layers, but these methods have a difficulty in providing uniform film thickness.